It is often necessary or desirable to measure various blood constituents, such as blood gases, pH, other electrolytes and glucose. This can be accomplished in real time using various sensors, for example electrochemical sensors and fluorescent sensors. This can be accomplished in an extracorporeal blood loop as shown in Cooper U.S. Pat. Nos. 4,640,820 and in vivo as disclosed in Lubbers et al Reissue No. 31,879. For in vivo sensing, a probe or catheter carrying an appropriate sensor is inserted into a blood vessel of the patient.
One of the most important gases that needs to be sensed is oxygen. One problem with in vivo oxygen sensing is that the readings obtained for the concentrations of oxygen tend to vary over an unacceptably wide range when compared with the results obtained using conventional laboratory techniques for measuring the concentration of oxygen. Carbon dioxide concentrations and pH determinations also tend to vary relative to values obtained using conventional laboratory techniques. It has been found that such deviations are in many cases unacceptably large so that the reliability of the in vivo measuring system is called into question.